The present invention relates to honeycomb structures used as catalyst supports, and more particularly to ceramic honeycomb structures provided with protective coatings to improve the resistance of the structures to erosion or other physical damage during manufacture or while in service as catalyst carriers in exhaust systems adapted to purify combustion engine exhaust gases.
The exhaust gases emitted by internal combustion engines utilizing hydrocarbon fuels such gasoline or diesel fuel normally include a number of pollutants including unburned hydrocarbons, carbon monoxide, and nitrogen oxides (NOx). The automotive industry has developed complex catalytic exhaust treatment systems designed to substantially remove most of these harmful constituents from the engine exhausts.
Channeled honeycomb catalyst supports, typically in the form of extruded ceramic honeycombs composed predominantly of cordierite, have long been preferred for use as substrates to support catalytically active components effective to successfully treat engine exhaust streams. Cordierite is a particularly preferred material on account of its low thermal expansion, high thermal shock resistance, and high refractoriness. The production of cordierite (2MgO.2Al2O3.5SiO2) ceramics from mineral batches containing sources of magnesium, aluminum and silicon such as clay and talc is well known. U.S. Pat. No. 3,885,977, for example, discloses the manufacture of thermal-shock-resistant cordierite honeycomb ceramics from clay/talc batches by extruding the batches and firing the extrudates to provide honeycomb structures with very low thermal expansion coefficients.
Manufacturers work continuously to optimize the characteristics of cordierite substrates to enhance their utility as catalyst carriers. One important advance has been the development of so-called thin-walled cordierite honeycombs. These are honeycombs formed with channel walls of substantially decreased thickness, generally not exceeding 125 μm in wall thickness and more typically in the range of 20-100 μm in wall thickness. Demand for honeycomb catalyst supports having very thin cell walls is increasing in response to legislation requiring higher conversion efficiencies in catalytic converters for the automobile market. Thinner cell walls reduce the mass of the substrates, resulting in faster catalyst light-off times. In addition, higher geometric surface areas may be achieved without an increase in the mass of the substrate, and without an increase in exhaust gas back pressures that can degrade engine efficiency. Examples of cordierite honeycomb structures with ultra-thin cell walls are disclosed, for example, in U.S. Pat. No. 6,506,336.
One disadvantage associated with the use of these very thin-walled ceramic honeycombs is that they are subject to chipping or erosion due to the relative fragility of ceramic material of slight thickness. Chipping can be caused by handling of the honeycombs during the various processes of applying catalyst coatings to the channel walls; erosion can occur in later use as the inlet ends of the honeycombs are impacted by particles entrained in the exhaust gas streams passing through the structures. The problem of erosion is in fact magnified in those automotive exhaust treatment applications where the catalytic converter is moved closer to the engine exhaust manifold.
It has been generally recognized that the protective treatment of the inlet faces of such ceramic catalyst supports can help to address these problems. Examples of some of the proposed honeycomb treatments are disclosed in U.S. Pat. Nos. 4,294,806, 6,242,072, and 6,352,756. Other approaches are disclosed in published Japanese patent applications JP 54-71791 and JP 2004-000907.
However, up to the present time, problems with existing approaches remain, and no truly satisfactory solution has been provided. Among the remaining problems are those of chemical and/or thermal incompatibilities between the ceramic honeycombs and the protective coating materials that may be applied to the honeycomb end faces. Any successful protective coating must adhere well to honeycomb surfaces over a broad range of temperatures, and must not interfere with the favorable high temperature properties of the honeycombs or the performance of subsequently applied catalyst coatings. Another problem is that of cost. Coating processes that require high temperature firing steps to cure or bond protective coatings to the honeycombs are uneconomic, and also risk physical or chemical degradation of the underlying cordierite or other ceramic wall structures. Yet another problem is that of increased engine exhaust back pressures arising from the use of thick protective coatings that decrease honeycomb channel diameters and thereby increase exhaust gas flow pressure drops across the honeycombs.